Image intensifier tubes are well known in the industry by their commonly used names based on the generic generation from which their design came into being. The tubes have evolved from Generation 0 to Generation III.
A significant portion of the military and commercial night vision equipment currently in use was designed to physically accommodate a Generation II (Gen II) image intensifier tube. One particular piece of equipment which is in widespread use is an AV/VVS-2 driver's viewer shown in FIG. 1a. An exploded view of the driver's viewer is shown in FIG. 1b. A driver's viewer 10 is a periscope type of arrangement used on military vehicles such as tanks. It incorporates a Generation II 25 mm Electro-Static Inverter image intensifier tube 12 with a fiber optic magnifier 14 enclosed in a housing 16. The housing 16 has a nominal length "A" of 130 mm and a nominal width "B" of 72 mm. An objective lens assembly 18 is positioned in front of the tube 12. The Gen II tube and magnifier are positioned in the viewer between a window/mirror assembly 20 and a biocular eyepiece 22 through which the driver of the vehicle views the scene ahead.
The output of the Gen II tube 12 is directed to an image plane which is spaced the precise distance in front of the biocular eyepiece required to transmit the desired image to the user. The image is, in this case, at the end surface of the fiber optic magnifier.
The Gen II tube conforms to a very detailed U.S. military specification and is identified by its U.S. military part number: MX-9644. Referring to FIG. 2 there is shown the tube 12 and the fiber optic magnifier 14. A photocathode 24 is positioned in one end of the tube 12 and a power supply 26 surrounds the tube. This type of tube is well known in the industry and specific details of its structure are not repeated here.
The Gen II tube is an inverter type tube and together with the magnifier assembly exhibits a gain of 15,000 or greater at 6.times.10.sup.-6 foot candles input. The photocathode sensitivity comprises luminous sensitivity at 2856.degree. K. of about 325 microamps per lumen. The Gen II tube exhibits a signal-to-noise ratio of approximately 4:1 and a resolution of 28 line pairs per millimeter (1 p/mm).
The Gen II tube has some shortcomings. These include optical losses with the fiber optic magnifier and incompatibility with the laser-blocking Coated Optical Components (COC).
A Generation III (Gen III) image intensifier device exhibits many advantages over the Gen II tube. The device employs a gallium arsenide photocathode with improved photosensitivity extending response into the near infrared range which in turn extends operation to starlight levels and below. It exhibits a Gain in the range of 20,000-35,000 at 2.0.times.10.sup.-6 foot candles. The sensitivity of the photocathode at 2856.degree. K. is about 1000 microamps per lumen which is nearly three times that of the Gen II tube. The signal-to-noise ratio has quadrupled to approximately 16:1. In addition, the resolution has been increased to 36-40 1 p/mm.
Replacement of the Gen II tube with a Gen III tube is an advantage in military and commercial applications due to the greatly improved capability of the Gen III tube relative to the Gen II tube.
Besides its increased performance, the Gen III tube has increased tube life. It is thus desirable to replace the Gen II tubes with the improved Gen III tubes. However, the Gen II tube cannot easily be replaced by the Gen III tube.
One major reason for the difficulty in substituting Gen III tubes for Gen II tubes is the shorter length of the Gen III tube. Such substitution would require substantial modification of the housing in which the tube is mounted. This in turn would necessitate costly redesign and replacement of the housing as well as redesign of the viewing system in which the housing is located.
Another problem resides in the fact that the Gen II tube has an inverting output, and the Gen III tube has a non-inverting output.
The Gen III based intensifier replacement must be as close as possible to a form, fit and function replacement of the Gen II tube in order for the upgrading to be cost effective.
Prior methods of enhancing night vision system performance by using a Gen III tube in place of a Gen II tube and fiber optic magnifier, while maintaining the same housing dimensions, have included: i) combining a Gen III tube with a Gen I 25 mm image intensifier and a fiber optic 25 mm-to-46 mm magnifier, and ii) combining a Gen III tube with a Gen I 25 mm-to-46 mm image intensifier.
Both methods provide an inverted output and more importantly both have the overall dimensions to be accommodated into the Gen II housing of the driver's viewer. However, each of these solutions has deficiencies.
The first "solution", as shown in FIG. 3, includes a Gen III image intensifier tube 30 which has a photocathode 32, a microchannel plate 34 and a fiber optic output window 36. A Gen I image intensifier tube 38 is positioned adjacent the Gen III tube with its fiber optic input window 40 joined to the output window 36 of the Gen III tube. The Gen I tube 38 also inverts the image. The fiber optic output window 42 of the Gen I tube 38 is joined to a fiber optic magnifier 44. A housing 46 surrounds the two tubes, magnifier and a power supply 48.
The output of the magnifier 44 is relayed by the driver's viewer eyepiece to the user exactly as in the Gen II system.
The second "solution," as shown in FIG. 4, requires a Gen III tube 50 and a 25 mm-to-46 mm Gen I magnifier tube 52. The Gen III tube 50 has an photocathode 54, a microchannel plate 56 and a fiber optic output window 58.
The Gen I tube has a fiber optic input window 60 and a fiber optic output window 62. A power supply 64 provides power to both the Gen I and Gen III tubes. As in FIG. 3, the output window 58 of the Gen III tube is joined to the input window 60 of the Gen I tube.
There are several disadvantages to these solutions. The fiber optic magnifier of solution 1 introduces 25 significant optical losses, thus degrading the viewer's performance, as improved by the Gen III technology, to an unacceptable level. Additionally, the fiber optic magnifier is a relatively expensive component.
The Gen I magnifier tube is also expensive. Moreover, the use of two vacuum envelopes results in a shorter mean time between failures. The power supplies for both solutions must provide 12-15 kV for the Gen I tube and are complicated and expensive. The resolution degrades generally with the use of two image intensifiers and more particularly at the interface between the two image intensifiers. The assembly and packaging and alignment of the tubes is difficult and time consuming.
Also, Gen I image intensifier tubes are becoming scarce and, when available, are expensive. This is particularly true of the 25:46 magnifier tube.
It is therefore an object of the present invention to provide an improved image intensifier system for a driver's viewer which eliminates the use of a Generation I tube.
It is yet another object of the present invention to provide a Gen III tube-based intensifier system using a Gen II compatible housing.
It is an additional object of the present invention to provide a Gen III tube intensifier system which has improved sensitivity and gain relative to previously employed Gen II systems.
It is another object of the present invention to provide an image intensifier system which has a longer average life.
It is a still further object to provide an improved image intensifier system for a driver's viewer which is more economical and easier to fabricate.
It is another object of the invention to use a Gen III tube to generate the output image for the object plane of an eyepiece viewing assembly while maintaining the housing structure of the Gen II image intensifier tube.